Hemitoroidal resonator gyroscope

ABSTRACT

A method for fabricating a vibratory structure gyroscope is provided herein. An annular cavity is formed in a first surface of a substrate, the annular cavity defining an anchor post located in a central portion of the annular cavity. A bubble layer is formed over the first surface of the substrate and over the annular cavity. The substrate and the bubble layer are heated to form a hemitoroidal bubble in the bubble layer over the annular cavity. A sacrificial layer is deposited over the hemitoroidal bubble of the bubble layer and an aperture is formed in the sacrificial layer, the aperture disposed over the anchor post in the annular cavity. A resonator layer is deposited over the sacrificial layer and the sacrificial layer between the bubble layer and the resonator layer is removed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/112,551, filed on May 20, 2011, which claims the benefit of priorityto U.S. Provisional Application No. 61/349,876, filed on May 30, 2010,the disclosure of which is incorporated herein by reference.

BACKGROUND

Vibrating structure gyroscopes detect rotation by sensing changes in thevibration of a vibrating structure such as a resonator. Typically, theresonator is induced to vibrate by some mechanism. Rotation of thevibrating resonator can cause changes in the vibration (e.g., angle,speed). These changes can be detected and used to determine the rotationof the resonator. Vibrating structure gyroscopes typically includepiezoelectric, hemispherical, tuning fork, and wheel-type gyroscopes.

Hemispherical type vibrating structure gyroscopes can include aresonator having a hemispherical shell attached to a stem. The stem ofthe resonator is mounted to a structure, and the resonator can vibrateabout the stem. These types of gyroscopes can be made using conventionalhigh-precision machining techniques. It is typically desirable to buildthis type of gyroscope using micromachining techniques, allowing batchfabrication of many gyroscopes at one time.

In some examples, the uniformity of the hemispherical shell can affectthe accuracy with which the vibrations in the hemispherical shell can beused to detect rotation. In addition, the positioning of the stem on thehemispherical shell (e.g., if the stem is off centered) can also affectthe accuracy of the gyroscope. Thus, precision in the manufacture of thehemispherical type gyroscope is an important factor that can bedifficult to achieve.

SUMMARY

A method for fabricating a vibratory structure gyroscope is providedherein. An annular cavity is formed in a first surface of a substrate,the annular cavity defining an anchor post located in a central portionof the annular cavity. A bubble layer is formed over the first surfaceof the substrate and over the annular cavity. The substrate and thebubble layer are heated to form a hemitoroidal bubble in the bubblelayer over the annular cavity. A sacrificial layer is deposited over thehemitoroidal bubble of the bubble layer and an aperture is formed in thesacrificial layer, the aperture disposed over the anchor post in theannular cavity. A resonator layer is deposited over the sacrificiallayer and the sacrificial layer between the bubble layer and theresonator layer is removed.

DRAWINGS

Understanding that the drawings depict only exemplary embodiments andare not therefore to be considered limiting in scope, the exemplaryembodiments will be described with additional specificity and detailthrough the use of the accompanying drawings, in which:

FIG. 1A is a perspective view of a hemitoroidal resonator for avibrating structure gyroscope according to one embodiment;

FIG. 1B is another perspective view of the hemitoroidal resonator ofFIG. 1A;

FIG. 1C is a top view of the hemitoroidal resonator of FIG. 1A;

FIG. 2 is a cross-sectional side view of a hemitoroidal resonatorgyroscope according to one embodiment;

FIG. 3A is an illustration of a vibratory mode for a hemitoroidalresonator according to one embodiment;

FIG. 3B is an illustration of precession of a vibratory mode for ahemitoroidal resonator according to one embodiment;

FIGS. 4A-4M are cross-sectional side views of various stages in afabrication process for a hemitoroidal resonator gyroscope according toone embodiment;

FIG. 5 is a cross-sectional side view of a hemitoroidal resonatorgyroscope fabricated according to the process of FIGS. 4A-4M; and

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize specific features relevantto the exemplary embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments. It is to beunderstood that other embodiments may be utilized and that logical,mechanical, and electrical changes may be made. Furthermore, the methodpresented in the drawing figures and the specification is not to beconstrued as limiting the order in which the individual steps may beperformed. The following detailed description is, therefore, not to betaken in a limiting sense.

FIGS. 1A, 1B, and 1C illustrate a resonator 100 having a hemitoroidalshape for use in a vibrating structure gyroscope according to oneembodiment. The resonator 100 has an outer surface 110 and acorresponding inner surface 112 that follows the contours of the outersurface 110. The resonator 100 can protrude inwardly at a centralportion thereof to form a central stem 114 with an aperture 116. Thestem 114 can provide a point of attachment for the resonator 100 to anunderlying structure. An outer lip 118 of the resonator 100 surroundsthe stem 114. As shown in FIG. 1B, the hemitoroidal shape of resonator100 results in an inward depression 120 in the central portion of theouter surface 110. This inward depression 120 forms the stem 114 asillustrated in FIG. 1A.

In one example, the resonator 100 can be composed of a glass materialsuch as silica (e.g., amorphous SiO2). In other examples, thehemitoroidal resonator 100 can be composed of another glass materialhaving a low coefficient of thermal expansion (CTE). A low CTE can helpreduce thermoelastic damping of the resonator 100. Thermoelastic dampingcan define the fundamental limit to the quality factor Q of theresonator 100. Thermoelastic damping can occur when compressive andtensile stresses produce heating and cooling within the material of theresonator 100. This can induce a temperature gradient across thethickness of the resonator 100. The resulting heat flow can dissipatethe mechanical energy of the resonator 100. The rate of thermoelasticdamping can be proportional to the square of the CTE; therefore, in someexamples it can be advantageous to construct the resonator 100 of amaterial with a low CTE. In some examples, a few percent of titania(amorphous TiO₂) can be included in the silica material formingresonator 100 to reduce the CTE. A CTE near zero can be achieved whenthe titania concentration is about 7%. In other examples, the resonator100 can be composed of another material that provides high mechanicalquality factor, such as silicon, diamond, etc.

The hemitoroidal shape enables the resonator 100 to have excellentvibratory characteristics while also enabling the resonator 100 to beprecisely fabricated in a micro-electro-mechanical systems (MEMS) scalechip. Notably, the hemitoroidal shape can be fabricated with a highdegree of symmetry about its central axis of symmetry.

In one embodiment, the stem 114 of the resonator 100 is integrallyformed in the central portion of the hemitoroid. This enables the stem114 to be aligned with respect to the outer lip 118 with high precision,since the stem 114 and resonator 100 are formed in the same step(s).This also enables the resonator 100 to have increased strength since thestem 114 is an integral part of the resonator 100. Notably, theincreased precision and strength of the resonator 100 are due in part tothe fact that the stem 114 and the resonator 100 are not fabricated inseparated steps and then connected together. Additional detail regardingthe fabrication process is provided with respect to FIGS. 4A-4Mdescribed hereafter.

In one exemplary embodiment, the stem 114 can have an outer diameter ofabout 10-400 microns, while the outer lip 118 can have a diameter ofabout 0.5-4 mm (e.g., about 2 mm). The resonator 100 can have a materialthickness of about 3-30 microns.

Although the hemitoroidal resonator 100 in FIGS. 1A, 1B, and 1C is shownhaving a specific height, diameter, radius of curvature, stem width, andaperture size, it should be understood that other sizes of these partsof the hemitoroidal resonator 100 can be used. Moreover, in someexamples the stem 114 may be solid and thus may not include an aperture116.

FIG. 2 is a cross-sectional side view of a hemitoroidal resonatorgyroscope 200 having resonator 100 therein. In this example, thegyroscope 200 is a MEMS scale gyroscope. Accordingly, the gyroscope 200can be fabricated on a single wafer using semiconductor fabricationprocesses. The gyroscope 200 can include a substrate 210 having a topworking surface 212. The stem 114 of the resonator 100 can be attachedto the top working surface 212 of the substrate 210. The outer lip 118of the resonator 100 is separated from the surface 212 such that theresonator 100 can vibrate freely about the stem 114.

The substrate 210 can define an annular cavity 214 in the top workingsurface 212. An anchor post 216 of the substrate 210 can extend througha central portion of the annular cavity 214. In an example, additionalsemiconductor conductor components can be fabricated on the top workingsurface 212 of the substrate 210. The stem 114 of the resonator 100 canbe attached to the anchor post 216. The substrate 210 can include asilicon wafer, a gallium arsenide wafer, silica, silicon carbide, aglass-ceramic (e.g., Zerodur® a zero expansion glass-ceramicmanufactured by Schott AG), or other materials.

FIGS. 3A and 3B illustrate a top profile view of a vibratory mode forthe hemitoroidal resonator 100. As shown, the resonator 100 can beresonant in an n=2 quadrupole mode. In FIG. 3A, the outer lip 118 of thehemitoroidal resonator 100 is shown with instantaneous displacementoutward along the x-axis, and inward along the y-axis. One-halfvibration cycle later in time, the displacement would be outward alongthe y-axis and inward along the x-axis. In FIG. 3B, precession of theorientation of n=2 quadrupole vibratory mode is shown. The mode is shownrotated by angle theta due to external applied rotation Ω. The rotationvector is along the z-axis (out of plane of the paper). The angle thetaincreases with time, that is, the mode precesses about the z-axis.

FIGS. 4A-4M illustrate various stages in an example fabrication processfor the hemitoroidal resonator gyroscope 200. The fabrication processcan begin with a substrate 210 (e.g., a silicon wafer).

FIG. 4A depicts the substrate 210 after patterning and etching (e.g.,using a photomask) of the top working surface 212 to form an annularcavity 214 and an anchor post 216. In one example, etching to form theannular cavity 214 can include deep reactive ion etching (DRIE). Theanchor post 216 can be formed during the same patterning and etchingstep (e.g., using the same photomask) as used to form the annular cavity214. Therefore, the outer dimension of the annular cavity 214 can beformed using the same patterning and etching step as the anchor post216. This enables the positioning accuracy (e.g., the alignment withrespect to the outer dimension) of the anchor post 216 to be as good asthe positioning accuracy of the mask-writing process. That is, thepositioning accuracy is not dependent upon the alignment accuracy of apatterning step (e.g., one patterning step trying to align a featurewith a feature produced in another patterning step).

In one example, mask writing can be done with a spot size/tolerancesmaller than 0.05 microns. In a conventional hemispherical resonator,similar alignment would likely be done by photolithographic alignment,wafer bonding, or some other process having an alignment tolerance onthe order to 1 to 2 microns, a factor of 20-40 times worse than for thepresent hemitoroidal resonator. In one embodiment, the anchor post 216can be about 200 microns in diameter.

In an example, the depth of the annular cavity 214 can be determined inorder to achieve a desired hemitoroidal bubble as explained below.Assuming ideal gas behavior, the depth of the annular cavity 214 can becalculated as

${D = {\frac{\pi \; r}{4}\left( \frac{P_{A}T_{B}}{{P_{B}T_{A}} - {P_{A}T_{B}}} \right)}},$

where r is the radius of the toroidal tube, P_(B) and T_(B) are the gaspressure and temperature, respectively, at which the glass wafer 310(See FIG. 4B) is bonded to the substrate 210, and P_(A) and T_(A) arethe gas pressure and temperature, respectively, at which the wafer isannealed in FIG. 4C below.

In some examples, patterning and etching can also form other features onthe substrate 210. Additionally, in some examples, the substrate 210 canbe moderately conductive. Accordingly, a heavily doped silicon wafer canbe used or dopant can be diffused into the surface 212 (not shown) afteretching the annular cavity 214.

FIG. 4B illustrates a bubble layer 310 on the substrate 210. In anexample, the bubble layer 310 can initially comprise a glass wafer. Theglass wafer can be anodically bonded to the patterned and etched surface212 of the substrate 210. The glass wafer can be composed of materialssuch as Corning Pyrex, Corning Eagle XG, Schott Borofloat, or Hoya SD2.The glass wafer can be thinned by lapping and polishing to a desiredthickness (e.g., about 10-100 microns). In an example, the glass wafercan be thinned after bonding to substrate 210; however, in anotherexample, the glass wafer can be thinned before bonding to the substrate210. In another example, the glass wafer can be thinned by etching.After the glass wafer has been bonded to the substrate 210, the glasswafer can be etched to form an aperture 312 over the anchor post 216. Inan example, the aperture 312 can have a tapered edge that is smallercloser to the anchor post 216.

The subsequent steps of the process described hereafter show a bubbleexpanded outward in the glass wafer. In order to expand the glass wafer,gas of a certain pressure can be present in the annular cavity 214. Inan example, the pressure of gas used is a function of the properties ofthe glass wafer, the temperature in the annular cavity 214 before andafter the gas is expanded, etc. In order to achieve the desired pressureand composition in the annular cavity 214, the chamber housing thesubstrate 210 during anodic bonding can be filled with the desiredpressure and composition of gas. A variety of gases can be used, forexample, nitrogen, argon, air, etc. could be used, at a typical pressureof 0.2 to 1 atmosphere. In any case, a density of gas should be presentsuch that when expanded “blows” a bubble in the glass wafer such thatthe bubble reaches a desired size and ceases to expand.

FIG. 4C illustrates a hemitoroidal bubble 314 formed in the bubble layer310. In an example, the hemitoroidal bubble 314 is substantiallysymmetric around the anchor post 216. The bubble 314 can be formed byheating the substrate 210 and bubble layer 310 beyond the softeningpoint of the bubble layer 310. For example, the substrate 210 and thebubble layer 310 can be heated to about 850 degrees Celsius for Pyrexglass, about 975 degrees Celsius for Eagle XG or SD2 glass, or about1700 degrees Celsius for fused silica.

As the temperature rises, the pressure in the cavity will exceed theexternal pressure, causing the gas in the annular cavity 214 to expandand form the bubble 314 in the softened glass over the annular cavity214. Surface tension can help to form the bubble 314 with asubstantially constant radius of curvature such that the bubble 314 issymmetrical about the anchor post 216. The temperature can be controlledto control expansion of the gas and the resulting size of the bubble314. Once the bubble 314 has reached the desired size, the bubble 314can be allowed to harden by reducing the temperature at a ratesufficiently fast that the bubble does not collapse before hardening.The cooling rate can depend on the glass thickness, volume of theannular cavity, size of the bubble 314, etc. Once hardened thehemitoroidal bubble 314 can become the template for the remainder of theresonator 100.

Since the hemitoroidal shape of the bubble 314 forms naturally as aresult of heating the gas within the annular cavity 214, the bubble 314can be formed with high precision. Since the bubble 314 is a templatefor the hemitoroidal resonator 100, forming the bubble 314 with highprecision also results in a high precision resonator 100. In addition,since both the stem 114 and the outer lip 118 of the resonator 100 areformed using the bubble 314, the alignment of the stem 114 with respectto the outer lip 118 is based on the precision of the bubble 314. Asmentioned above, the precision of the bubble 314 is a natural result ofthe annular cavity 214 and the anchor post 216. Thus, the precision ofthe resonator 100 including the alignment of the stem 114 with respectto the outer lip 118 is based on the precision with which the annularcavity 214 and the anchor post 216 can be formed. Since, as mentionedabove, the annular cavity 214 and the anchor post 216 are formed usingthe same patterning and etching steps, the precision of the annularcavity 214 and the anchor post 216 can be quite high (e.g., having atolerance smaller than 0.05 microns). Accordingly, the resonator 100 canbe formed with high precision.

FIG. 4D shows a first conductive layer 320 deposited on the bubble layer310. The first conductive layer 320 can then be patterned to form aplurality of electrodes. In an example, the plurality of electrodes canextend from an outside edge of the bubble layer 310 towards a topportion of the bubble 314. The plurality of electrodes can form aplurality of extended portions of the conductive layer 320. Theplurality of electrodes can be used to drive and balance the resonator100. In an example, the first conductive layer 320 can be composed ofpolycrystalline silicon (“polysilicon”) and the plurality of electrodescan be formed by ion implantation all the way through the polysiliconfollowed by a rapid anneal.

In other examples, the first conductive layer 320 can be composed ofother materials including gold, nichrome, chromium, indium tin oxide(ITO), doped titania silicate glass, and the like. Although FIG. 4Dillustrates the plurality of electrodes primarily on the outside of thehemitoroidal bubble 314, in other examples, the plurality of electrodescan be patterned on other locations of the bubble 314 as well as onstructures (not shown) other than the bubble 314.

FIG. 4E illustrates a sacrificial layer 324 deposited on the firstconductive layer 320. The sacrificial layer 324 can be patterned andetched to form an aperture 326. Similar to the bubble layer 310, anaperture 326 in the sacrificial layer 324 can be formed over the anchorpost 216. In an example, the aperture 326 can have a tapered edge thatis smaller closer to the anchor post 216.

The patterning and etching of the sacrificial layer 324 can also be usedto form contacts (not shown) and other features (e.g., in the field324). In some examples, patterning can be performed with a projectiontype of lithography such as e-beam lithography or a stepper.

In an example, the sacrificial layer 324 can be composed of polysilicon(e.g., when the first conductive layer 320 is composed of a materialother than polysilicon). In other examples, the sacrificial layer 324can be composed of a rapidly etching glass or one of several selectivelyetching metals such as titanium (Ti), molybdenum (Mo), Chromium (Cr),and the like. In yet other examples, the sacrificial layer 324 can becomposed of silicon nitride (Si₃N₄), molybdenum alloys (Moly), aerogel,polyimide, or parylene. In still other examples, the sacrificial layer324 can be composed of multiple layers of material. For example, if thefirst conductive layer 320 is composed of polysilicon, a first layer ofthe sacrificial layer 324 can be composed of metal (Ti, Mo, Cr, etc.)with a thicker layer of polysilicon on top.

In an example, a low pressure chemical vapor deposition (LPCVD) is usedto form the sacrificial layer 324. In another example, a plasma enhancedchemical vapor deposition (PECVD) process can be used to formsacrificial layer 324. In addition, multiple depositions can be employedto produce a thicker sacrificial layer 324. The deposition conditionscan be varied to give balanced stress in the sacrificial layer 324. Inan example, the sacrificial layer 324 can have a thickness of about 5-20microns.

FIG. 4F shows a second conductive layer 330 deposited on the sacrificiallayer 324. The second conductive layer 330 can then be patterned to formone or more electrodes for the inner surface 112 of the resonator 100.In an example, the one or more electrodes can be thin (e.g., less than200 angstroms) with respect to the resonator layer 334 in order toreduce the impact of the electrode materials on the thermoelasticdamping of the resonator.

In an example, the second conductive layer 330 can be composed ofpolysilicon. In other examples, the second conductive layer 330 can becomposed of other materials including gold, nichrome, chromium, indiumtin oxide (ITO), doped titania silicate glass, and the like.

FIG. 4G illustrates a resonator layer 334 deposited over the secondconductive layer 330. In one example, the resonator layer 334 can becomposed of a glass material such as silica (e.g., amorphous SiO2). Inother examples, the resonator layer 334 can be composed of another glassmaterial having a low coefficient of thermal expansion (CTE). A low CTEcan help reduce thermoelastic damping of the resulting resonator. Insome examples, a few percent of titania (amorphous TiO₂) can be includedin the silica material of the resonator layer 334 to reduce the CTE. ACTE near zero can be achieved when the titania concentration is about7%. In other examples, the resonator layer 334 can be composed ofanother material that provides high mechanical quality factor, such assilicon, diamond, etc.

In one example, the resonator layer 334 can have a thickness of about 10microns. In some examples, the resonator layer 334 can have a variablethickness such that the resulting resonator is thinner at the outer lip118 and thicker elsewhere for robustness. In an example, standarddeposition techniques can be utilized in forming the resonator layer334, such as chemical vapor deposition (CVD), low pressure CVD (LPCVD),atmospheric pressure CVD (APCVD), thermal oxidation, sputtering,tetraethyl orthosilicate (TEOS) plus anneal, PECVD plus anneal, and lowtemperature oxide (LTO).

In one example, an oxide sputter deposition rate can be about 70angstroms per minute, while the sputter target surface can get depletedof one component such as silicon. Also, the substrate 210 can be heatedto an appropriate temperature (e.g., 400° C.) such that the resonatorlayer 334 forms an amorphous glass coating. In an example, the resonatorcan be designed to transmit little stress to the anchor post 216 at anaperture through the anchor post 216. For example, the radius (e.g.,width) of the stem 114 of the hemitoroidal resonator can be formed to begreater than the radius of the aperture in the anchor post 216.

FIG. 4H depicts the resonator layer 334 patterned and etched to producethe aperture 116 in the stem 114. In an example, the aperture 116 in thestem 114 can be used for coupling of a third conductive layer 338 to thesecond conductive layer 330 as shown in FIG. 4I. The aperture 116 can beformed with tapered edges wherein the diameter of the aperture 116 issmaller closer to the anchor post 216.

FIG. 4I shows the third conductive layer 338 deposited on the resonatorlayer 334. The third conductive layer 338 can then be patterned to formone or more electrodes for the outer surface 110 of the resonator 100.In an example, a continuous electrode is formed for the outer surface110 of the resonator 100. Thus, for a continuous electrode, patterningmay be used only in the field 342 adjacent to the hemitoroidal bubble314 to form contacts, leads, etc. In an example, the one or moreelectrodes can be thin (e.g., less than 200 angstroms) relative to theresonator layer 334 in order to reduce the impact of the electrodematerials on the thermoelastic damping of the resonator 100.

The third conductive layer 338 can be composed of polysilicon and theone or more electrodes can be formed by ion implantation followed by arapid anneal. In other examples, the third conductive layer 338 can becomposed of other materials including gold, nichrome, chromium, ITO,doped titania silicate glass, and the like. In an example, the thirdconductive layer can comprise a tungsten film this deposited by CVD toproduce a uniform metal film for a conductor on the resonator 430. TheCVD-deposited tungsten can also form a barrier layer for etchant of thesacrificial layer (as described with respect to FIG. 4K.

In an example, one or more pad layers (not shown) can also be formed forwire bonding. Standard materials and techniques can be used to form thepad layers.

FIG. 4J illustrates a photoresist layer 340 over the resonator layer334. The photoresist layer 340 can be patterned in order to remove thephotoresist layer 340 except over the hemitoroidal bubble 314. That is,after patterning and etching, the remaining portion of the photoresistlayer 340 can cover the portion of the resonator layer 334 that willresult in the resonator 100.

In an example, a multi-step photoresist can be used. For example, afirst layer 340-1 of photoresist that is thin and relatively uniformthickness over the entire surface of the bubble 314 can be deposited by,for example, spraying a mist of small droplets of photoresist. Thisfirst photoresist layer 340-1 can be exposed and developed to remove theresist in the areas approximately parallel to the surface 212 of thesubstrate 210. That is, exposing and developing the first photoresistlayer 340-1 can remove the photoresist 340-1 in areas such as the topsof the hemitoroidal bubble 314 and a flat field 342 adjacent thehemitoroidal bubble 314. The exposure, however, can be limited such thatthe photoresist 340-1 on the slope of the hemitoroidal bubble 314 canremain with a sufficient thickness. In an example, the exposure can becontrolled such that the photoresist 340-1 covers the bottom edge (e.g.,where the outer lip 118 will be formed) and ends cleanly at the boundarybetween the bubble 314 and the flat field 342.

This process can be used to define the outer lip 118 of the resonator100. This process can enable tight control of the definition of theouter lip 118 since the photoresist edge is automatically aligned withthe future outer lip 118 of the resonator 100 by using the curvature ofthe bubble 314 to remove portions of the photoresist on the field 342,but leave portions of the photoresist on the bubble 314 wherever thesurface of the bubble 314 is approximately perpendicular to the surface212.

Once the first photoresist layer 340-1 is exposed, a second photoresistlayer 340-2 can be printed over the first photoresist layer 340-1. In anexample, this second photoresist layer 340-2 can be relatively crude inalignment as long as the photoresist 340-2 covers most of the bubble 314without covering an edge 344 of the first photoresist layer 340-1. Thatis, the outer lip 118 of the resulting resonator 100 can be definedprimarily by the first photoresist layer 340-1 instead of the secondphotoresist layer 340-2.

In FIG. 4K, the third conductive layer 338, the resonator layer 334, andthe second conductive layer 330 can be etched using directional etches.In an example, ion milling can be used to etch. In another example,reactive ion etching can be used. The directional etching in combinationwith the defined edge 344 of the photoresist layer 340 can produce a cutinto the third conductive layer 338, the resonator layer 334, and thesecond conductive layer 330 that is aligned with the outer lip 118 ofthe resulting resonator 100.

In FIG. 4L, the sacrificial layer 324 can be exposed and etched away.For example, if the sacrificial layer 324 is polysilicon, a liquidetchant such as ethylenediamine pyrocatechol (EDP) or a gaseous etchantsuch as XeF₂ can be used. In some examples, the gaseous etchant isdesirable since it can reduce “stiction” problems that arise from theuse of liquids around compliant structures. There are, however, othertechniques for reducing stiction (e.g., self-assembled monolayers,freeze drying, etc.) that can be used when using a liquid etchant.

In an example, if the sacrificial layer 324 comprises multiple differentlayers, the thicker polysilicon can be etched first, with the metalprotecting the underlying polysilicon electrode from the etchant. Thenthe metal can be removed, exposing the electrodes of the firstconductive layer 320. Suitable sacrificial etchants can include hotphosphoric acid for Si₃N₄, H₂O₂ for Molybdenum, XeF₂ for polysilicon,oxygen plasma for polyimide, and buffered oxide etch (BOE) for aerogel.In an example, the etchant selections can allow etching of thesacrificial layer several microns thick on about a 1 millimeter radiushemitoroid without attacking the device layers.

At FIG. 4M, the photoresist layer 340 is removed. In an example, liquidsolvents can be used to remove the photoresist layer 340. In anotherexample, dry techniques (e.g., O₂ plasma ashing) can be used to removethe photoresist layer 340.

Although an example fabrication process is described above, it should beunderstood that other alternative processes can be utilized infabricating a hemitoroidal resonator gyroscope. For example, instead ofattaching the hemitoroidal resonator 100 to the anchor post 216directly, in another example, the resonator 100 can be attached to thebubble layer 310 above the anchor post 216. That is, the bubble layer310 may not have the aperture 312 etched therein. Without the aperture312 in the bubble layer 310, the resonator 100 can attach to the bubblelayer 310 instead of the anchor post 216.

In another example, a hemitoroidal resonator can be formed with aninward bubble extending into a cavity in a substrate. For example, acavity having a hemitoroidal shape can be formed in a substrate. Thecavity can form a hemitoroidal template for the resonator. An isotropicsilicon etchant can be used to produce the sloped sidewalls for thehemitoroidal cavity. Once the hemitoroidal cavity is formed, a resonatorlayer can be bonded to the substrate in a vacuum such that a vacuum isformed in the hemitoroidal cavity. The substrate and bubble layer canthen be heated in a pressurized chamber such that a bubble is formed inthe resonator layer that extends into the annular cavity. Forcing thebubble into the annular cavity can form a more robust anchor post forthe hemitoroidal resonator. Electrodes can then be formed on thesidewall of the annular cavity. Other steps in the fabrication processcan be similar to that described with respect to FIGS. 4A-4M.

In yet another example, a resonator layer can be placed on the substrateand a bubble can be formed in the resonator layer directly. For example,a low CTE glass, such as Corning ULE® glass, for use as a resonator canbe directly bonded to a substrate. Inward or outward bubbles could thenbe formed directly in the low CTE glass. In some examples, the softeningpoint of the low CTE glass can be above the melting point of silicon, soan alternate starting substrate having a higher melting temperature canbe used along with corresponding alternate techniques for etching.Example substrate materials for the alternate substrate include SiC,tungsten (W), and Ti.

In an example, a bubble can be formed directly in the resonator layer bythe following process. A substrate (e.g., composed of SiC, tungsten, andTi) can be etched (e.g., using DRIE) to form an annular cavity and ananchor post. A resonator layer (e.g., SiO₂) can be bonded to thesubstrate. The resonator layer can be thinned to a desired thickness ofthe resonator. The substrate and the resonator layer can then be heatedto the softening point of the resonator layer to form the bubble in theresonator layer. In some examples, the heating can be inductive heatingand/or incremental laser heating such as a CO₂ laser beam directedthrough the open end of a furnace tube. A patterned sacrificial layer(e.g., Si₃N₄ and/or Molybdenum) on the substrate can be used to releasethe outer lip of the resonator. In some examples, the outer lip of theresonator can be heated with a laser or by blackbody radiation toproduce a thicker wall at the outer lip.

FIG. 5 illustrates a cross-sectional side view of a hemitoroidalresonator gyroscope 400 fabricated according to the process of FIGS.4A-4M. The gyroscope 400 can include a substrate 410, such as a siliconwafer, that defines an annular cavity 412. An anchor post 414 of thesubstrate 410 can extend through a central portion of the annular cavity412. A bubble layer 416, composed of Pyrex glass for example, can beattached to an upper surface of the substrate 410. The bubble layer 416can include a hemitoroidal bubble 418 over the annular cavity 412. Afirst conductive layer 420, composed of polysilicon for example, andcomprising a plurality of electrodes can be disposed over the glasslayer 416. A pad layer 422 can be disposed on a field portion of theconductive layer 420.

A hemitoroidal resonator 430 can be disposed over the hemitoroidalbubble 418 and annular cavity 412. In an example, the resonator 430 canbe composed of a glass material with thin conductive layers 436 and 438on the inner surface and the outer surface. The resonator 430 caninclude a stem 432 in a central portion thereof that is attached to theanchor post 414 of substrate 410. The stem 432 can include a stepstructure 433 formed as a result of the apertures in the bubble layer416 and/or the sacrificial layer. The outer lip 434 of the resonator 430can be positioned slightly apart from the conductive layer 420 such thatthe resonator 430 is allowed to vibrate freely about the stem 414.

The second conductive layer 436 comprising a plurality of electrodes onan inner surface (e.g., inner surface 112) of the resonator 430. Thethird conductive layer 438 on an outer surface (e.g., outer surface 118)can be continuous.

In an alternative embodiment, the electrodes can be replaced by combfingers (e.g., protrusions on an outer lip of the resonator 430) formedon the outer lip 434 of the resonator 430. The comb fingers can bedefined by patterning the photoresist (e.g., photoresist 340-1) on theflat field laterally adjacent to the outer lip 434 lip of the resonator430. Then, the resonator layer can be etched to form the combs, and thecombs can be coated with the third conductive layer 438.

While electrodes can be used for sensing the vibration of the resonator430, in other examples, an optical sensor can be used to sense thevibration of the resonator 430. Thus, in an example, the drive andpickoff of the gyroscope 400 can be electrical and optical,respectively. In another example, the drive and pickoff can both beoptical.

In another embodiment, the plurality of electrodes for driving theresonator 430 and sensing vibration (e.g., formed from the conductivelayer 420) can be disposed outside of the hemitoroidal resonator 430.For example, the plurality of electrodes can be disposed on one or moreadditional bubbles formed on the substrate 410 laterally adjacent to theresonator 430.

In a further embodiment, the plurality of electrodes can be disposed onan electrode support structure (not shown) above the resonator 430. Inthis case, the second conductive layer 436 on an inner surface (e.g.,inner surface 112) of the resonator 430 can be continuous and the thirdconductive layer 438 on an outer surface (e.g., outer surface 118) cancomprise a plurality of electrodes. Additionally, one or more aperturescan be formed in the electrode support structure in order to enable anetchant to quickly pass through for etching of a sacrificial layerbetween the resonator 430 and the electrode support structure. Anelectrical contact on the stem 434 of the resonator 430 can mechanicallyconnect the resonator 430 to the electrode support structure. Theelectrode support structure can be composed of doped titania silicateglass to reduce thermal expansion mismatch.

In an example operation of the gyroscope 400, the hemitoroidal resonator430 can be driven to resonance based on electrical signals provided tothe first plurality of electrodes of the conductive layer 420. Theelectrical signals provided to these electrodes can induce movement inthe second plurality of electrodes 436 on the inner surface of theresonator 430. In an example, this can cause n=2 quadrupole moderesonance in the resonator 430 as shown in FIG. 3A. When the gyroscope400 is rotated the orientation of the n=2 quadrupole vibratory modeprecesses about the symmetry axis of the hemitoroid, i.e., the centralaxis of the anchor post 414. The angle of precession is a measure of theintegrated angle of the applied rotation. The precession can be sensedbased on electrical property changes between the second plurality ofelectrodes 436 and the first plurality of electrodes of the conductivelayer 420. Based on these sensed electrical property changes,information can be provided from the gyroscope via the pads 422 to otherelectrical components. The other electrical components can thendetermine a rotation of the gyroscope based on this information. Inanother example, the mode precession can be sensed optically, bymeasuring the time-varying obstruction of light beams caused by thevibratory motion of the n=2 quadrupole mode. In another example, thegyroscope 400 can operate in a force rebalance mode, in which rebalancevoltages are applied to electrodes to produce forces on the resonator430 that prevent the precession of the n=2 quadrupole mode when thegyroscope 400 is rotated. In this example, the rebalance voltages are ameasure of the applied rotation rate.

According to examples described herein, a hemitoroidal resonator can befabricated with sufficient precision for a high performance gyroscope.In some examples, the majority of the vibration for the hemitoroidalresonator can occur near the outer lip of the resonator. A measure ofperformance of a rate-integrating vibratory gyroscope is the angulargain factor, defined as the angle of rotation of the vibrating modepattern measured with respect to the rotating resonator, divided by theangle of external applied rotation. In an example, the angular gainfactor for the hemitoroidal resonator gyroscope can be about 0.58compared to an angular gain factor of about 0.3 for a conventionalhemispherical resonator gyroscope. Thus, the hemitoroidal resonator canreduce sources of error since the signal to noise ratio is about two (2)times higher than that of a conventional hemispherical resonatorgyroscope.

Additionally, the stem of the resonator can be aligned with highprecision. That is, a high degree of azimuthal symmetry (e.g.,positioning of the stem with respect to the outer lip) can be achieved.In an example, the stem on the resonator can be positioned moreprecisely by a factor of around 20 times as compared to a conventionalhemispherical resonator. This increased precision results in bettersymmetry for the resonator, which can improve performance. Additionally,the hemitoroidal resonator permits less costly, batch fabrication on awafer.

Any of the example hemitoroidal gyroscopes discussed herein can beincorporated into an electronic device for obtaining rotational sensinginformation therefrom. For example, an electronic device can include ahemitoroidal gyroscope coupled to one or more processing devices whichare coupled to one or more memory devices. The one or more memorydevices can include instructions which, when executed by the one or moreprocessing devices, can cause the one or more processing devices toperform one or more acts. In an example, the one or more processingdevices can include a microprocessor, a microcontroller, a digitalsignal processor, field programmable gate array (FPGA), etc. The one ormore memory devices can include any appropriate processor readablemedium used for storage of processor readable instructions or datastructures.

The processor-readable media can be implemented as any available mediathat can be accessed by a general purpose or special purpose computer orprocessor, or any programmable logic device. Suitable processor-readablemedia can include tangible media, such as storage or memory media, andtransmission media such as electrical, electromagnetic, or digitalsignals, conveyed via a communication medium such as a network and/or awireless link.

Storage or memory media can include magnetic or optical media, such asconventional hard disks, Compact Disk-Read Only Memory (CD-ROM),volatile or non-volatile media such as Random Access Memory (RAM)(including, but not limited to, Synchronous Dynamic Random Access Memory(SDRAM), Double Data Rate (DDR) RAM, RAMBUS Dynamic RAM (RDRAM), StaticRAM (SRAM), etc.), Read Only Memory (ROM), Electrically ErasableProgrammable ROM (EEPROM), and flash memory, etc.

In one example, the instructions can cause the one or more processingdevices to receive information from the hemitoroidal gyroscope and usethe information for inertial measurement purposes.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiments shown. Therefore, it ismanifestly intended that this invention be limited only by the claimsand the equivalents thereof.

What is claimed is:
 1. A method of fabricating a vibratory structuregyroscope, the method comprising: forming an annular cavity in a firstsurface of a substrate, the annular cavity defining an anchor postlocated in a central portion of the annular cavity; forming a bubblelayer over the first surface of the substrate and over the annularcavity; heating the substrate and the bubble layer to form ahemitoroidal bubble in the bubble layer over the annular cavity;depositing a sacrificial layer over the hemitoroidal bubble of thebubble layer; forming an aperture in the sacrificial layer, the aperturedisposed over the anchor post in the annular cavity; depositing aresonator layer over the sacrificial layer; and removing the sacrificiallayer between the bubble layer and the resonator layer.
 2. The method ofclaim 1, further comprising: depositing a first conductive layer overthe hemitoroidal bubble in the bubble layer before depositing thesacrificial layer; etching the first conductive layer to form aplurality of electrodes; depositing a second conductive layer over thesacrificial layer; and etching the second conductive layer to form anelectrode layer.
 3. The method of claim 2, further comprising:depositing a third conductive layer over the resonator layer.
 4. Themethod of claim 1, further comprising: depositing a first photoresistlayer over the resonator layer; exposing and developing the photoresistlayer to remove portions of the first photoresist layer that areparallel to the first surface of the substrate; and printing a secondphotoresist layer over a portion of the resonator layer corresponding tothe hemitoroidal bubble, wherein an edge of the first photoresist layercorresponding to an outer lip of a resonator is left uncovered by thesecond photoresist layer.
 5. The method of claim 4, further comprising:etching the resonator layer using directional etching in a directionperpendicular to the first surface of the substrate.
 6. The method ofclaim 5, wherein etching the resonator layer includes etching with agaseous etchant.
 7. The method of claim 1, further comprising: etchingan aperture in the bubble layer over the anchor post prior to depositingthe sacrificial layer.
 8. The method of claim 7, wherein etching anaperture in the bubble layer includes forming tapered edges of theaperture having a smaller diameter closer to the anchor post.
 9. Themethod of claim 1, further comprising: thinning the bubble layer to athickness in a range of about 10 microns to 100 microns prior to heatingthe substrate and bubble layer.
 10. The method of claim 1, whereinforming the annular cavity and the anchor post includes etching theannular cavity and the anchor post using the same photomask.
 11. Themethod of claim 1, wherein heating includes forming a bubble expandingoutward from the annular cavity.
 12. The method of claim 11, whereinheating includes expanding a gas in the annular cavity such that apressure in the annular cavity exceeds a pressure external to theannular cavity.
 13. The method of claim 1, wherein heating includesforming a bubble extending into the annular cavity.
 14. A method offabricating a vibratory structure gyroscope, the method comprising:forming an annular cavity in a first surface of a substrate, the annularcavity defining an anchor post located in a central portion of theannular cavity; forming a resonator layer over the first surface of thesubstrate and over the annular cavity; and heating the substrate and theresonator layer to form a hemitoroidal bubble in the resonator layerover the annular cavity.
 15. The method of claim 14, wherein heatingincludes forming an inward bubble extending into the annular cavity. 16.The method of claim 15, wherein forming a resonator layer includesbonding the resonator layer to the substrate such that a vacuum isformed in the annular cavity; and wherein heating includes heating in apressurized chamber.
 17. The method of claim 15, wherein forming anannular cavity includes etching to produce a sloped sidewall for theannular cavity.
 18. The method of claim 15, comprising: depositing aconductive layer on the sloped sidewall; and etching the conductivelayer to form a plurality of electrodes.
 19. The method of claim 14,wherein heating includes forming an outward bubble expanding outwardfrom the annular cavity.